In the manufacture of integrated circuits (IC), patterns representing different layers of the IC are fabricated using a series of reusable photomasks (“masks”) to transfer the design of each layer of the IC onto a semiconductor substrate during the manufacturing process in a photolithography process. These layers are built up using a sequence of processes and resulted in transistors and electrical circuits. However, as the IC sizes continue to shrink, meeting accuracy requirements as well as reliability in multiple layer fabrication has become increasingly more difficult.
Photolithography uses an imaging system that directs radiation onto the photomask and then projects a shrunken image of the photomask onto a semiconductor wafer covered with photoresist. The radiation used in the photolithography may be at any suitable wavelength, with the resolution of the system increasing with decreasing wavelength. Deep ultraviolet (DUV) light with a radiation at a wavelength of 248 or 193 nanometers (nm) has been widely used for exposure through a transmissive mask. However, with the shrinkage in IC size, extreme ultraviolet (EUV) lithography with a typical wavelength of 13.5 nm becomes one of the leading technologies for 16 nm and smaller node device patterning.
An EUV mask utilized for the EUV lithography is a layered structure including a Bragg mirror deposited on a substrate. On the substrate, a reflective multilayer stack, which is formed by sequentially stacking materials having different optical properties, is used to achieve a high EUV light reflectance. The pattern is formed from absorptive features or lines etched into the EUV mask. The reflective multiplayer stack is a type of Bragg reflector that reflects light at a selected wavelength through constructive interference. The thicknesses of the alternating layers are tuned to maximize the constructive interference (Bragg reflection) of the EUV light reflected at each interface and to minimize the overall absorption of the EUV light. The multiplayer coating can achieve about 60 to 75% reflectivity at the peak radiation wavelength. The EUV Lithography process may lack spectral purity for its light sources, meaning the light sources may produce undesirable out-of-band (OoB) radiation, i.e., radiation of an undesirable bandwidth, for example, between 193 nanometers (nm) to 257 nm. Existing photoresist materials may be sensitive to the OoB radiation and may absorb such radiation. This would result in reduced contrast and hence degradation of imaging performance.
On the other hand, the EUV masks require frequent cleaning to reduce or eliminate defects during the optical lithography operation. The cleaning is typically performed at an elevated temperature to enable and/or enhance the efficiency of the cleaning chemistry. In addition, during use the masks are inadvertently heated through exposure with extreme ultraviolet light. In this regard, the mask is frequently exposed to temperatures above ambient during the masks lifecycle and is used at temperatures exceeding ambient during normal operation. Consequently, these conditions can cause several types of chemical diffusion and chemical reactions within the multilayer stack of the Bragg mirror.